1. Field of the Invention
The invention relates to a developing apparatus for developing an electrostatic image formed on an image bearing member by an electrophotographic printing method or an electrostatic recording method, and particularly to a developing apparatus provided with a valve for the supply and discharge of a developer.
2. Related Background Art
In image forming apparatuses, and above all, a color image forming apparatus of an electrophotographic printing type which performs color image formation, there has heretofore been widely utilized a dual-component developing method using a mixture of a toner and a carrier as a developer.
On the other hand, in a full-color copying machine, printer or the like available in recent years, the introduction of the following technique has been advanced as a method of maintaining a high quality of image without performing the work of interchanging a developer.
For example, Japanese Patent Publication No. JP-B-H02-021591 discloses an image forming apparatus in which a carrier supplying device and a toner supplying device are provided separately from or integrally with each other above agitating means for agitating a carrier and a toner. In this image forming apparatus, there is adopted a construction in which a developer spillover portion is provided in a side wall of the housing of a developing apparatus and a fresh developer is supplied little by little by a supplying device and is discharged from the developer spillover portion.
That is, in the image forming apparatus of the above-described construction, a deteriorated carrier in the developer is gradually replaced with a fresh one, whereby the apparent progress of the deterioration of the carrier is stopped, and a characteristic is stabilized as the whole of the developer. Thereby, the work of interchanging the developer is made unnecessary and a maintenance property is improved.
As described above, in a developing apparatus which has taken long-term image stability into account, it is popular to provide a developer supplying port and a developer spillover portion (hereinafter referred to as the “developer discharging port”).
However, an inconvenience arises in a so-called rotary developing apparatus wherein such a developing apparatus is provided in a system, for example, a developing rotary in which the developing apparatus itself is moved, for example, is rotatably carried.
That is, at the other positions than a position at which the developing apparatus is opposed to a photosensitive member, and the developing apparatus is maintained in a substantially horizontal position, there is the possibility that the developer may inadvertently leak out from the above-mentioned supplying port and discharging port. In such a case, the developer in the developing apparatus greatly diminishes with a result that developer coat uniformity on an image bearing member is spoiled or no coat is applied at all. Accordingly, there arises the inconvenience that density uniformity becomes faulty or no density is provided at all.
Therefore, it is conceivable to provide a valve in the developer supplying port or the discharging port, to prevent the developer from inadvertently leaking out from the supplying port or the discharging port with the exception that the developing apparatus is in a substantially horizontal position, and to control the supply and discharge of the developer.
The above-mentioned valve, however, is located at a position where it is liable to be stained with the developer and therefore, the operation thereof has often become unstable. Particularly, in a low-humidity environment, the discharging amount of the toner per unit mass increases. Therefore, it becomes difficult to remove the toner adhering to a stopper wall for regulating the operation range of the valve by a Coulomb force. In a state in which such a toner is sandwiched between the stopper wall and the valve for a long period, there has been the possibility that the toner may adhere the valve to the stopper wall in the fashion of a binder to thereby hamper the function of the valve.